1. Field of the Invention
This invention relates to quality assurance methods used for quality assurance for laser shock peening and, more particularly, for natural frequency monitoring and analysis method for quality assurance of a production laser shock peening process.
2. Discussion of the Background Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477, entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957, entitled “Material Properties”. Laser shock peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or uncoated surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled “Laser Peening System and Method”. These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP'D to create the beneficial compressive residual stresses. Other techniques to confine and redirect the shock waves that do not use water have also been developed.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. Nos.: 5,756,965 entitled “ON THE FLY LASER SHOCK PEENING”; 5,591,009, entitled “Laser shock peened gas turbine engine fan blade edges”; 5,569,018, entitled “Technique to prevent or divert cracks”; 5,531,570, entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; 5,492,447, entitled “Laser shock peened rotor components for turbomachinery”; 5,674,329, entitled “Adhesive tape covered laser shock peening”; and 5,674,328, entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock peening.
LSP is a deep treatment of the material and it is desirable to have a quality assurance test that is indicative of a volumetric LSP effect. It is also desirable to have a QA method that is compatible with a dual sided or simultaneous dual sided LSP process wherein substantially equal compressive residual stresses are imparted to both sides of a workpiece, i.e. along the leading edge of a gas turbine engine fan blade.
One laser shock peening quality assurance technique previously used is high cycle fatigue (HCF) testing of blades having leading edges which are LSP'd and notched in the LSP'd area before testing. This method is destructive of the test piece, fairly expensive and time consuming to carry out, and significantly slows production and the process of qualifying LSP'd components. An improved quality assurance method of measurement and control of LSP that is a non-destructive evaluation (NDE), inexpensive, accurate, quick, and easy to set up is highly desirable. It is also desirable to have a real time NDE quality assurance method that is relatively inexpensive and sufficiently economical to be used on each workpiece instead of a sampling of workpieces. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. It is desirable to have a real time NDE method so that process deviations can be discovered during a production run. Therefore, any real time techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.
Interferometric profilometry method and apparatus to obtain volumetric data of a single laser shock peened test dimple created with a single firing of a laser used in the laser shock peening process is disclosed in U.S. Pat. No. 5,948,293, entitled “Laser shock peening quality assurance by volumetric analysis of laser shock peened dimple”. Other QA methods are disclosed in U.S. Pat. No. 5,987,991, entitled “Determination of Rayleigh wave critical angle”; U.S. Pat. No. 5,974,889, entitled “Ultrasonic multi-transducer rotatable scanning apparatus and method of use thereof”; and U.S. Pat. No. 5,951,790, entitled “Method of monitoring and controlling laser shock peening using an in plane deflection test coupon”. U.S. Pat. No. 6,254,703, entitled “Quality Control Plasma Monitor for Laser Shock Processing” discloses a method and apparatus for quality control of laser shock processing by measuring emissions and characteristics of a workpiece when subjected to a pulse of coherent energy from a laser. U.S. patent application Ser. No. 09/969,744, filed Oct. 3, 2001, and entitled “LASER SHOCK PEENING QUALITY ASSURANCE BY ACOUSTIC ANALYSIS” discloses a method for quality control of laser shock processing by measuring an acoustic signal for each laser beam pulse, calculating an acoustic energy parameter value for each of the acoustic signals, and calculating a statistical function value of the workpiece based on the acoustic energy parameter values. These empirically measured emissions and characteristics of the workpiece are correlated to theoretical shock pressure, residual stress profile, or fatigue life of the workpiece. These methods typically use a radiometer or acoustic detection device for measuring these characteristics.